This invention relates to electrochromic devices, and particularly, to an electrochromic device requiring minimal external electrical power for coloration and in which substantially no polarization occurs.
Electrochromic devices are well-known devices which exhibit a phenomenon known as "persistent electrochromism", e.g., see U.S. Pat. No. 3,521,941 entitled, "Electro-Optical Device Having Variable Optical Density," issued July 28, 1970. The term "persistent electrochromism" denotes the property of a material whereby its electromagnetic radiation absorption characteristic is altered, in most instances, even at ambient temperature, under the influence of an electric field. Such materials, for example, may exhibit little or no absorption of visible wavelength in the absence of an electric field, and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, turning blue in color. Similar effects can be observed in other portions of the electromagnetic spectrum, invisible as well as visible.
As described in the prior art, if a layer of a persistent electrochromic material is disposed between a pair of electrodes across which a potential is applied, the radiation transmitting characteristic of the material will change. If the electrodes and the electrochromic material are formed on the surface of a transparent substrate, such as glass, the light transmitting characteristics of the combination can be varied by controlling the electric field produced across the electrochromic material. For example, application of a voltage between the electrodes to establish an electric field of the proper polarity changes the light absorption characteristics of the electrochromic material, turning it darker, for example, thus decreasing the light transmitting ability of the entire assembly.
The phenomenon of "persistent electrochromism" has also been exhibited in electrochromic devices which include an electrolyte-electrochromic sandwich wherein the electrolyte functions both as a conductive medium and as a source of positive ions. For example, the sulfuric acid electrolyte of U.S. Pat. No. 3,708,220, issued Jan. 2, 1973. In these devices, the electrolyte is chosen as to be sufficiently conductive to permit low voltage operation of the electrochromic device while also being chemically compatible with the electrochromic layer and electrode employed in the device.
Although electrochromic devices have been developed and are successful for many applications, most electrochromic devices include electrochromic layers which require external electrical power for both coloration and for bleaching. Some electrochromic devices can be bleached by merely short circuiting the electrodes, but these devices still require external electrical power for coloration.
Furthermore, due to the electrochemical reactions occurring in conventional electrochromic devices, it has been found that the electrode adjacent to the electrolyte often becomes polarized. Polarization, when present, limits the switching speed, especially at contrasts in excess of 2:1, since the current decreases sharply with time. For example, aquadag, a conductive mixture of graphite and a non-conductive binder, is often employed as an electrode for electrochromic devices, since it is found that the graphite is capable of removing electrons from the negative ions in the electrolyte, thereby permitting a current flow. However, it appears that the aquadag electrode is responsible for polarization in the electrochromic device.
Although not wholly understood, it is presently believed that the removal of electrons from the negative ions in the electrolyte often creates a barrier of uncharged neutral species at the electrode. For example, with H.sub.2 SO.sub.4 utilized as the acid electrolyte, the positive graphite electrode attracts the negative SO.sub.4 .sup.= radical and removes electrons from the negative SO.sub.4 .sup.= so as to leave a neutral SO.sub.2 as well as O.sub.2 species at the electrode. It is also suspected that negative OH.sup.- radicals may be attracted to the positive graphite electrode wherein these unstable OH.sup.- radicals react to form H.sub.2 O and O.sub.2 at the positive electrode. The neutral SO.sub.2 and O.sub.2 species can thereafter present a barrier to a flow of charge, thereby slowing down the switching speed or response time of the electrochromic device. Whether or not the above mechanisms prove to govern in these devices, the observed decrease in current with time, by definition, is caused by polarization. It would therefore be desirable to develop an electrochromic device which requires no external electrical power and one in which substantially no polarization occurs at the electrode adjacent the electrolyte.